1. Field of the Invention
The present invention relates to a frequency characteristics-variable amplifying circuit for amplifying a high-frequency signal and a semiconductor integrated circuit device mounted with this frequency characteristics-variable amplifying circuit.
2. Description of the Related Art
An optical communications receiving module is mounted with a photo-detector to convert a light signal to an electric signal and a broadband amplifier into which an electric signal outputted from this photo-detector is inputted and which rectifies a signal waveform attenuated and distorted during a transmission through an optical fiber. This broadband amplifier is composed of an amplifier mainly for amplifying an attenuated signal and an amplifier for mainly rectifying a waveform disorder. Recently, with the realization of downsized and low-cost optical communications modules, broadband amplifiers wherein the respective amplifiers are formed as an integrated circuit on a single semiconductor substrate by use of a bipolar integrated circuit-manufacturing technique have been developed (see Japanese Patent Laid-Open Publication No. 88087/1999 and Specification of U.S. Pat. No. 6,340,899 B1).
As an example of an amplifying circuit provided in such a prior-art broadband amplifier, description will be given of an amplifying circuit as described in U.S. Pat. No. 6,340,899 B1. FIG. 1 is a circuit diagram showing a prior-art frequency characteristics-variable amplifying circuit as described in U.S. Pat. No. 6,340,899 B1. As shown in FIG. 1, this prior-art amplifying circuit 50 is connected to power supply potential wiring and ground potential wiring. And, in the amplifying circuit 50, from the power supply potential wiring to the ground potential wiring, an inductor 51, a resistor 53, an output terminal 55, and an NMOS transistor 59 are series-connected in this order, and in parallel with these, an inductor 52, a resistor 54, an output terminal 56, and an NMOS transistor 60 are series-connected in this order. In addition, to gates of the NMOS transistor 59 and the NMOS transistor 60, an input terminal 61 and an input terminal 62 are connected, respectively, and sources of the NMOS transistor 59 and the NMOS transistor 60 are connected to the ground potential wiring via a transistor 63. Furthermore, to a gate of this transistor 63, a bias terminal 64 to which a bias potential is applied is connected. Herein, a load capacitance 57 and a load capacitance 58 shown in FIG. 1 each show parasitic capacitance, which inevitably occurs between this amplifying circuit 50 and ground potential wiring.
Next, operations of this prior-art amplifying circuit 50 will be described. First, a power supply potential is applied to the power supply potential wiring, a ground potential is applied to the ground potential wiring, and a bias potential is applied to the transistor 63. In this condition, for example, when complementary signals are inputted so that the input terminal 61 becomes high and the input terminal 62 becomes low, the NMOS transistor 59 is turned on, and the NMOS transistor 60 is turned off. Thereby, a low signal is outputted from the output terminal 55, and a high signal is outputted from the output terminal 56.
In a case of this prior-art amplifying circuit 50, an output signal voltage is a potential difference between the output terminal 55 and the output terminal 56, and the greater this potential difference is, the greater the gain becomes. FIG. 2 is a graph showing frequency characteristics of a gain where the horizontal axis shows a frequency and the vertical axis shows a gain. As shown in FIG. 2, in general, when an amplifying circuit is used, peaking of its gain occurs in a high-frequency region. In terms of the amplifying circuit 50 shown in FIG. 1, since the inductor 51 and inductor 52 are provided within the circuit, the peaking value becomes great in the gain frequency characteristics as shown in FIG. 2, and an output signal waveform can be sharpened.
However, since the prior-art amplifying circuit 50 cannot vary frequency characteristics of inputted signals, the amplifying circuit cannot control the peaking position and cannot correspond to frequencies other than a frequency presumed in design, therein exists a problem.